Fix firefox addon_success needle
    
    I updated this last month without looking closely enough; one of
    the matches was bogus. We now don't seem to get the 'added to
    Firefox' popup any more, so we can't match on it, but I guess we
    can match on the 'remove' button as an indicator that the add
    worked.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>